Different winding schemes for a toroidal inductor can affect its magnetic field leakage, which will intensify the near-filed coupling of an electromagnetic interference (EMI) filter. First, the effect of equivalent radial current and tangential current on magnetic field leakage was studied by using the partial element equivalent circuit method, and it was found that the equivalent current loop composed of tangential current was the main source of axial magnetic field leakage in the toroidal inductor. Then, a winding scheme was proposed, in which a PCB was sandwiched by two magnetic cores to generate reverse current loops above and below the PCB for magnetic field cancellation. In the experimental link, three-dimensional finite element simulation was conducted to demonstrate that the proposed scheme can further reduce the axial magnetic field leakage while maintaining a similar inductance as the traditional scheme. The impedance and insertion loss tests of the EMI filter were conducted, and it was verified that compared with the traditional scheme, the novel scheme can weaken the influence of high-frequency parasitic parameters on the inductor’s impedance characteristic, thereby further optimizing the high-frequency filtering performance of the EMI filter.

To address the power quality issues in a multiple grid-connected-converter system (MGCCS) for renewable energy, the series and parallel models of MGCCS are established at first based on the harmonic state space (HSS) method. The HSS models account for high-frequency components during the switching process, with its internal Toeplitz matrix effectively capturing the harmonic coupling within the converters. Based on the series and parallel HSS models of MGCCS, a virtual resistor is introduced to improve the model, and the impact of grid impedance is considered to simulate the multi-converter grid-connected system. In addition, the harmonic stability of this system is investigated by analyzing the eigenvalues of the state matrix. The influences of main circuit and controller parameters on the harmonic instability are analyzed in depth, and the oscillation frequencies in harmonic instability modes are predicted. Simulation results based on MATLAB/Simulink and RT-LAB experimental results validate the accuracy of the improved HSS models and the harmonic stability analysis.

Voltage sag is one of the main power quality problems, and dynamic voltage restorers (DVRs) provide an effective solution. Since the traditional DVRs are unsuitable for medium-voltage applications, a high-voltage direct-connect DVR based on a cascaded H-bridge multilevel converter is introduced. To enhance the response speed and compensation accuracy of the system, a dq-axis decoupling strategy for the AC voltage compensation control of DVR is proposed. Meanwhile, a carrier phase shifted pulse width modulation strategy with voltage offset injection is also employed to balance the DC-side capacitor voltage of the cascaded H-bridge, thus ensuring the safety and stability of the system. Finally, the proposed DVR and the corresponding control algorithm are modeled and simulated by MATLAB/Simulink, demonstrating the effectiveness and feasibility of the approach.

Due to the intermittency of distributed generations and frequent fluctuations in non-linear loads in microgrids, significant changes in harmonic energy will occur. Although multiple parallel active power filters (APFs) can be used to effectively mitigate harmonics in microgrids, the parallel operation will result in circulating currents due to uneven compensation currents among the APFs, leading to increased system losses. To address this issue, a current-sharing control method based on the integrated transmission technology for electric energy information is proposed. With the incorporation of information modulation at the power modulation stage of an inverter, the transmission of key information is realized while the converting the electric energy, thereby enabling current-sharing control of the parallel system. Compared with the traditional centralized and master-slave control methods, the proposed method eliminates the need for additional communication equipment and lines, enhancing the system’s reliability, flexibility and scalability. The results of a simulation experiment based on RT-LAB validate the feasibility and effectiveness of the proposed current-sharing control method.

An accurate estimation of utility harmonic impedance is the key to harmonic source determination. Most of the existing methods are based on the premise that the background harmonics keep constant or fluctuate smoothly. However, the background harmonic voltage or utility harmonic impedance generally exhibits unsmooth fluctuation patterns in the novel power system, and there are even abrupt changes in some time sections, which leads to large calculation errors in the existing methods. To solve this problem, an estimation method for utility harmonic impedance considering the time-varying characteristics of background harmonics is proposed. First, this method identifies multiple abrupt change points of background harmonics by means of the binary sliding T maximum, and the measured harmonic data at a point of common coupling (PCC) is divided into several intervals of continuous unsmooth fluctuation accordingly. Then, with the consideration of the small variation of background harmonics between adjacent sampling points, utility harmonic impedance estimation is carried out based on the principle of comprehensive minimum fluctuation within each data interval, and the matrix sparse technique is introduced to reduce the computational time complexity. Finally, simulations and field data are adopted to conduct a comparative analysis, and results indicate that compared with the existing methods, the proposed method can more accurately estimate the utility harmonic impedance and demonstrate a higher computational efficiency.

With the continuous improvement of the degree of electronification in a power system, the proportion of harmonics in the system increases significantly. An accurate tracing of the source of harmonic disturbance is crucial for evaluating the influencing factors for power quality and providing important support for targeted control of harmonics. However, there are challenges in the large-scale harmonic tracing of power grids as follows. The sampling time of harmonic data of each power quality monitoring equipment is not synchronized, resulting in low time-domain accuracy and low effectiveness of the monitoring data. Through the calculation and analysis of the source tracing process, the amplification of errors will affect the quantitative results and source tracing effect. In a multi-harmonic source system, complex in multi-band impedance will affect the accurate traceability of a single disturbance source at a specific frequency. To solve the above problems, a harmonic state estimation method based on the time scale datum of different nodes is used to synchronously screen the monitoring data. With a focus on the complexity in the multi-band impedance characteristics, a multi-harmonic source tracing method based on multi-band impedance comparison at the same node is proposed. Through a comparison of the impedance results in multi-band at the same node, the potential harmonic source nodes can be effectively identified, and the harmonic force contribution of each node is calculated to quantitatively characterize its harmonic contribution. Finally, a simulation example of a multi-harmonic source power system including multiple new energy stations is established, and results fully verify the effectiveness of the proposed theoretical method.

With the "Dual-carbon" strategy proposed in China, the microgrid composed of distributed generations has become an effective way to solve the energy crisis and environmental pollution problems. To improve the power quality of a microgrid system, frequency restoration control is usually added to distributed generations to compensate for the frequency deviation caused by changes in load. It is necessary to obtain the information about accurate feeder impedance when calculating the load terminal frequency, so as to reduce the reliance of the system on high-speed communication. However, this is often difficult to be guaranteed in practical engineering, thus leading to the difference of power distribution among distributed generations and eventually deteriorating the frequency stability of the system. To solve these problems, the mechanism of an unstable system caused by the inaccurate feeder impedance is analyzed, and it is pointed out that the calculation error of load terminal frequency is the basic cause of power distribution difference among distributed generations. On this basis, a frequency restoration control strategy based on small-AC-signal injection is proposed. By adding an additional small-AC-signal to the output voltage from each distributed generation, a droop relationship between the injected small-AC-signal frequency and the calculated value of load terminal frequency is established, and the active power of the small signal is used to compensate for the calculated deviation of load terminal frequency caused by the inaccurate feeder impedance, so as to realize the compensation for frequency deviation while maintaining the power sharing of the system. To illustrate the application scope of this control strategy, the differential-mode small-signal model of a typical microgrid system is established, and the design method for related parameters is discussed. Finally, the effectiveness of the proposed method was verified by a hardware-in-the-loop experiment.

In response to the problem that the extraction accuracy of existing harmonic detection methods is easily affected by noises, a novel harmonic detection method based on iteration symplectic geometry mode decomposition combined with total least squares and estimation of signal parameters with rotation invariant technique (TLS-ESPRIT) is proposed. First, the initial symplectic geometric components are obtained by constructing a trace matrix of harmonic signals and combining the matrix transformation. Second, based on dynamic time warping, the initial symplectic geometric components with a high similarity are linearly combined, and the residual components are eliminated. Third, the normalized mean absolute error is calculated to obtain the independent harmonic components. Finally, the TLS-ESPRIT algorithm is used to obtain the parameters of different harmonic and inter-harmonic components. Through an analysis of simulation and measured data, the superiority of the proposed method when it is applied in harmonic and inter-harmonic analysis under a noisy environment is verified.

With the rapid development of new energy and power electronics technologies, the proportion of nonlinear loads in power systems is increasingly growing, leading to more severe power quality issues in power grid. The identification of load equivalent impedance parameters is one of the important methods for analyzing the characteristics of nonlinear loads. The status quo of researches on the load equivalent impedance parameter technology and its application fields is systematically sorted out, and the research directions and application trends in the future are forecasted. First, the basic principle for load equivalent impedance parameters is elaborated upon. Second, the existing load equivalent impedance parameter identification methods are analyzed, and the accuracy, applicable scope, advantages and disadvantages of different parameter identification methods are pointed out. Third, the applications of load equivalent impedance parameters in harmonic analysis and instantaneous reactive power calculation are reviewed. Finally, further research directions for load equivalent impedance parameters are discussed, and the prospects for extended applications beyond the fields at present are given.